Re: [KVM PATCH v3 4/4] kvm: add iosignalfd support

Marcelo Tosatti
2009-05-22T22:06:43Z

On Thu, May 21, 2009 at 12:51:29PM -0400, Gregory Haskins wrote:
> iosignalfd is a mechanism to register PIO/MMIO regions to trigger an eventfd
> signal when written to by a guest. Host userspace can register any arbitrary
> IO address with a corresponding eventfd and then pass the eventfd to a
> specific end-point of interest for handling.
>
> Normal IO requires a blocking round-trip since the operation may cause
> side-effects in the emulated model or may return data to the caller.
> Therefore, an IO in KVM traps from the guest to the host, causes a VMX/SVM
> "heavy-weight" exit back to userspace, and is ultimately serviced by qemu's
> device model synchronously before returning control back to the vcpu.
>
> However, there is a subclass of IO which acts purely as a trigger for
> other IO (such as to kick off an out-of-band DMA request, etc). For these
> patterns, the synchronous call is particularly expensive since we really
> only want to simply get our notification transmitted asychronously and
> return as quickly as possible. All the sychronous infrastructure to ensure
> proper data-dependencies are met in the normal IO case are just unecessary
> overhead for signalling. This adds additional computational load on the
> system, as well as latency to the signalling path.
>
> Therefore, we provide a mechanism for registration of an in-kernel trigger
> point that allows the VCPU to only require a very brief, lightweight
> exit just long enough to signal an eventfd. This also means that any
> clients compatible with the eventfd interface (which includes userspace
> and kernelspace equally well) can now register to be notified. The end
> result should be a more flexible and higher performance notification API
> for the backend KVM hypervisor and perhipheral components.
>
> To test this theory, we built a test-harness called "doorbell". This
> module has a function called "doorbell_ring()" which simply increments a
> counter for each time the doorbell is signaled. It supports signalling
> from either an eventfd, or an ioctl().
>
> We then wired up two paths to the doorbell: One via QEMU via a registered
> io region and through the doorbell ioctl(). The other is direct via iosignalfd.
>
> You can download this test harness here:
>
> ftp://ftp.novell.com/dev/ghaskins/doorbell.tar.bz2
>
> The measured results are as follows:
>
> qemu-mmio: 110000 iops, 9.09us rtt
> iosignalfd-mmio: 200100 iops, 5.00us rtt
> iosignalfd-pio: 367300 iops, 2.72us rtt
>
> I didn't measure qemu-pio, because I have to figure out how to register a
> PIO region with qemu's device model, and I got lazy. However, for now we
> can extrapolate based on the data from the NULLIO runs of +2.56us for MMIO,
> and -350ns for HC, we get:
>
> qemu-pio: 153139 iops, 6.53us rtt
> iosignalfd-hc: 412585 iops, 2.37us rtt
>
> these are just for fun, for now, until I can gather more data.
>
> Here is a graph for your convenience:
>
> http://developer.novell.com/wiki/images/7/76/Iofd-chart.png
>
> The conclusion to draw is that we save about 4us by skipping the userspace
> hop.
>
> --------------------
>
> Signed-off-by: Gregory Haskins <ghaskins@novell.com>
> ---
>
> arch/x86/kvm/x86.c | 1
> include/linux/kvm.h | 15 ++++
> include/linux/kvm_host.h | 10 ++-
> virt/kvm/eventfd.c | 165 ++++++++++++++++++++++++++++++++++++++++++++++
> virt/kvm/kvm_main.c | 11 +++
> 5 files changed, 198 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 98c2434..cee63ff 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1085,6 +1085,7 @@ int kvm_dev_ioctl_check_extension(long ext)
> case KVM_CAP_IRQ_INJECT_STATUS:
> case KVM_CAP_ASSIGN_DEV_IRQ:
> case KVM_CAP_IRQFD:
> + case KVM_CAP_IOSIGNALFD:
> r = 1;
> break;
> case KVM_CAP_COALESCED_MMIO:
> diff --git a/include/linux/kvm.h b/include/linux/kvm.h
> index 8f53f24..8162466 100644
> --- a/include/linux/kvm.h
> +++ b/include/linux/kvm.h
> @@ -292,6 +292,19 @@ struct kvm_guest_debug {
> struct kvm_guest_debug_arch arch;
> };
>
> +#define KVM_IOSIGNALFD_FLAG_DEASSIGN (1 << 0)
> +#define KVM_IOSIGNALFD_FLAG_PIO (1 << 1)
> +#define KVM_IOSIGNALFD_FLAG_COOKIE (1 << 2)
> +
> +struct kvm_iosignalfd {
> + __u64 cookie;
> + __u64 addr;
> + __u32 len;
> + __u32 fd;
> + __u32 flags;
> + __u8 pad[12];
> +};
> +
> #define KVM_TRC_SHIFT 16
> /*
> * kvm trace categories
> @@ -419,6 +432,7 @@ struct kvm_trace_rec {
> #define KVM_CAP_MCE 31
> #endif
> #define KVM_CAP_IRQFD 32
> +#define KVM_CAP_IOSIGNALFD 33
>
> #ifdef KVM_CAP_IRQ_ROUTING
>
> @@ -525,6 +539,7 @@ struct kvm_irqfd {
> _IOW(KVMIO, 0x74, struct kvm_assigned_msix_entry)
> #define KVM_DEASSIGN_DEV_IRQ _IOW(KVMIO, 0x75, struct kvm_assigned_irq)
> #define KVM_IRQFD _IOW(KVMIO, 0x76, struct kvm_irqfd)
> +#define KVM_IOSIGNALFD _IOW(KVMIO, 0x77, struct kvm_iosignalfd)
>
> /*
> * ioctls for vcpu fds
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 7dcae4b..5b2be86 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -138,6 +138,7 @@ struct kvm {
> struct kvm_io_bus pio_bus;
> #ifdef CONFIG_HAVE_KVM_EVENTFD
> struct list_head irqfds;
> + struct list_head iosignalfds;
> #endif
> struct kvm_vm_stat stat;
> struct kvm_arch arch;
> @@ -535,19 +536,24 @@ static inline void kvm_free_irq_routing(struct kvm *kvm) {}
>
> #ifdef CONFIG_HAVE_KVM_EVENTFD
>
> -void kvm_irqfd_init(struct kvm *kvm);
> +void kvm_eventfd_init(struct kvm *kvm);
> int kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags);
> void kvm_irqfd_release(struct kvm *kvm);
> +int kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args);
>
> #else
>
> -static inline void kvm_irqfd_init(struct kvm *kvm) {}
> +static inline void kvm_eventfd_init(struct kvm *kvm) {}
> static inline int kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
> {
> return -EINVAL;
> }
>
> static inline void kvm_irqfd_release(struct kvm *kvm) {}
> +static inline int kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
> +{
> + return -EINVAL;
> +}
>
> #endif /* CONFIG_HAVE_KVM_EVENTFD */
>
> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> index c63ff6a..7700e39 100644
> --- a/virt/kvm/eventfd.c
> +++ b/virt/kvm/eventfd.c
> @@ -21,12 +21,16 @@
> */
>
> #include <linux/kvm_host.h>
> +#include <linux/kvm.h>
> #include <linux/workqueue.h>
> #include <linux/syscalls.h>
> #include <linux/wait.h>
> #include <linux/poll.h>
> #include <linux/file.h>
> #include <linux/list.h>
> +#include <linux/eventfd.h>
> +
> +#include "iodev.h"
>
> /*
> * --------------------------------------------------------------------
> @@ -207,9 +211,10 @@ kvm_deassign_irqfd(struct kvm *kvm, int fd, int gsi)
> }
>
> void
> -kvm_irqfd_init(struct kvm *kvm)
> +kvm_eventfd_init(struct kvm *kvm)
> {
> INIT_LIST_HEAD(&kvm->irqfds);
> + INIT_LIST_HEAD(&kvm->iosignalfds);
> }
>
> int
> @@ -232,3 +237,161 @@ kvm_irqfd_release(struct kvm *kvm)
> irqfd_release(irqfd);
> }
> }
> +
> +/*
> + * --------------------------------------------------------------------
> + * iosignalfd: translate a PIO/MMIO memory write to an eventfd signal.
> + *
> + * userspace can register a PIO/MMIO address with an eventfd for recieving
> + * notification when the memory has been touched.
> + * --------------------------------------------------------------------
> + */
> +
> +struct _iosignalfd {
> + u64 cookie;
> + u64 addr;
> + size_t length;
> + struct file *file;
> + struct list_head list;
> + struct kvm_io_device dev;
> +};
> +
> +static int
> +iosignalfd_in_range(struct kvm_io_device *this, gpa_t addr, int len,
> + int is_write)
> +{
> + struct _iosignalfd *p = (struct _iosignalfd *)this->private;
> +
> + return ((addr >= p->addr && (addr < p->addr + p->length)));
> +}
> +
> +/* writes trigger an event */
> +static void
> +iosignalfd_write(struct kvm_io_device *this, gpa_t addr, int len,
> + const void *val)
> +{
> + struct _iosignalfd *iosignalfd = (struct _iosignalfd *)this->private;
> +
> + eventfd_signal(iosignalfd->file, 1);
> +}
> +
> +/* reads return all zeros */
> +static void
> +iosignalfd_read(struct kvm_io_device *this, gpa_t addr, int len, void *val)
> +{
> + memset(val, 0, len);
> +}

Gregory,

Can you explain the reasoning behind limiting the interface to
write-only ranges, with reads returning zero.

Is that because it fits the use cases in mind for iosignalfd?

Not that I have a better suggestion at the moment, just trying
to understand.

Also, the heavy-weight exit avoidance assumes that the action signalled
will be serviced by the qemu device model in a separate CPU, otherwise
there is no gain, is that correct?